Alloy steel



Patented Nov. 7, 1950 ALLOY STEEL 'William Charles Clarke, Jr., Dundalk,Md., as

signor to Armco Steel Corporation, a corporation of Ohio No Drawing.Application June 13, 1949,

' Serial No. 98,883

4 Claims. 1

The present application is a continuation-inpart of my copendingapplication, Serial No. 768,718, filed August 14, 1947, which in turn isa continuation-in-part of application, Serial No. 705,245, filed October23, 1946, and. the invention relates to high temperature. stainlesssteel and products and articles of the steel.

An object of my invention is the provision of austenitic stainless steelwhich, in addition to having many useful high temperature properties,has good low-temperature tensile strength and may be satisfactorilyworked.

Another object of this invention is that of providing austeniticstainless steel of the character indicated having highly satisfactoryand dependable resistance both to creep and stressrupture under load atelevated temperatures.

A further object of my invention is the provision of austeniticstainless steel of the character indicated which is substantiallyeconomical and satisfactory to produce.

A still further object of this invention is the provision of articlesand products made of steel of the character hereinbefore set forth.

Other objects in part will be obvious and in part pointed out more fullyhereinafter.

The invention, accordingly, consists in the combination of elements andin the features of composition, articles and products, asdescri'bedherein, the scope of the application of which is indicated in thefollowing claims.

As conducive to a clearer understanding of certain features of myinvention, it may be noted at this point that stainless steels ingeneral, whether they be of a chromium-nickel grade or of straightchromium grade, to some valuable extent possess the property of surfacestability at high temperatures and are useful where resistance tocorrosive attack in the presence of heat is of importance. Recently,however, demands for high temperature duty is frequently called upon,not only to serve with appreciable immunity from the attack of certain'corrosive agents, but to possess great strength and resistance to creepand'rupture while under load at high temperatures. There are times, too,where low-temperature strength and other properties of high temperaturesteels enter as an important consideration, as where the" steel is to beused in the form of high temperatur" turbine parts 2 which areintermittently operated so as to expose the metal to hot and coldconditions while under load. These diverse needs are not alwaysfulfilled by stainless steels of specific composition; for example, twoalloy steels of different compositions may be equally effective in theirresistance to oxidation at high temperatures, yet while heated maydisplay widely different properties, with regard to load-carryingability at high temperatures.

The conventional austenitic chromium-nickel stainless steels for themost part are seriously limited in their high temperature physicalproperties. They often creep while heated, creep being a plasticdeformation or stretch coupled with loss of strength ultimately leadingto rupture of the steel. These steels sometimes also introducedifficulties having to do with tensile strength at low temperatures. Ingeneral, it can be said that the chromium-containing marten'sitic orferritic stainless steels have greater tensile strength at lowtemperatures than .do those of austenitic grade. It might be expectedthat this relation of strength would be retained at high temperatures. Ifind, however, that this does not follow.

Still, it is well recognized that many of the conventional austeniticsteels are unsatisfactory, for meeting the standards of high temperatureuse, particularly where the steels are to be relied upon forwithstanding mechanical stress. Engineering requirements have so farexceeded the properties available that other alloys are being sought foruse while hot. The known creepresistant and stress-rupture-resistantalloys in general are notoriously difiicult to work or shape. In fact,they may have a variety of properties which promise satisfactoryperformance at high temperatures, yet do not respond favorably toworking operations applied thereto for achieving desired products. Thefinal products often are found to have imperfections resulting frominferior working properties of the metal. Also, where the products arenot of a corrosionresistant grade, high temperature operating conditionsoften will greatly accelerate the tendency to corrode;

An outstanding object of my invention, accordingly, is the provision ofhigh temperature austenitic stainless steel wrought products and thedamaging effects of corrosion and heat and of withstanding creep andrupture while subject to high mechanical stress at elevated temperature,and which have excellent low-temperature tensile strength and many otherhighly satisfactory low temperature properties.

Referring now more particularly to the practice of my invention, Iprovide high temperature chromium-nickel stainless steels whichimportantly contain substantial amounts of phosphorus, molybdenum andcopper. These steels, in addition to having excellent high temperatureproperties including resistance to stress-rupture and creep, haveexcellent physical properties at relatively low temperatures such as atroom temperature. temperature tensile strength and are readily forgedand worked such as by hot-cold forging to desired shape. In View of theability of the steels to resist stretch and other eifects of load athigh or low temperature, I frequently produce such products as turbinediscs thereof, these for operation at different temperatures prevailingin each body such as at relatively low temperatures in the center and athigher temperatures at the edge. The steels also have excellentrelaxation properties, relaxation being the ability to maintain a lowrate of loss of stress when under a fixed strain at high temperatures.Certain of the products which I produce of the steel, therefore, takethe form of bolts and high temperature fastening devices which have theproperty of remaining firm and tight in the initially seated position.

My stainless steels, or products and articles of the steel, moreparticularly contain approximately 0.01% to 0.15% carbon, 12% to 22%chromium, 8% to 21% nickel, from 0.05% to 0.5% phosphorus, 1.5% to 4%molybdenum, 2% to 4% copper, 0.10% to 2% manganese, and the remaindersubstantially all iron. Small amounts of other elements as forspecial purposes sometimes are present in the steel; however, I'findthat by avoiding the use of titanium and columbium, not

1 are sh ui tofmt'ial e n y savmg ad m 8 cos er S n the annealed steel,I -reheat'the metal, this time tering the alloy, but considerably betterlowtemperature strength is had-along with improved hot-cold forgingproperties; Also, in producing the steel substantially free of titaniumand columbium, pouring difiiculties arelessened. Columbium, andespecially titanium, give rise to various 'diificulties in teemingand'in the mold such as segregation and scumformation andt'en'd to givenitride and oxide stringers which, as during or after the steel has beenforged orrolled, impair strength and load carryingability. I, therefore,keep the steel substantiallyfree of titanium and columbium and of oxidesand nitrides of these elements.

My chromium-nickel-phosphorus stainless steel importantly issubstantially wholly austenitic. Ferrite, if present at all, is only intraces preferably not exceeding about 1.5% to 2% by volume. This virtualexclusion of ferrite, and the inclusion of critical amounts of carbon,phosphorus, copper, molybdenum and manganese in the steel, I findenables high temperature use at temperatures even so high as 1200 F. to1700 F. or more, and for resisting creep, relaxation and stress rupture.With a higher carbon content than within thecomposition'limits of mysteel, the working propertiessuifer and products of the metal are not soeasily produced. Likewise, with a manganese content substantiallyexceeding 2% Especially, the steels have excellent low 4 there is a lossin hot-working properties. With appreciable lowering of the phosphorus,molybdenum and copper content, the high temperature load-carryingcharacteristics of the steel suifer, and with appreciable increase,workability disappears.

The stainless steel and articles and products which I provide often areuseful without particular heat treatment or after annealing. I achieve aremarkable increase though in the high temperature load-carryingability, and also better lowtemperature properties, by employing heattreatment in thenature of an anne'aling and precipitation treatment. Inthe annealing operation,

"-1 heat the steel to a sufliciently high temperature and for longenough time to put at least part of the copper into solid solution. Whendesired, advantage is taken of the annealing temperatures to hot-workthe steel. A preferred temperature range for this purpose is that ofabout 2050 F. to 2250 F.

After the heating for solubility, I quench the steel preferably at aboutroom temperature, for example, while using air, oil or water as thequenching medium. This annealed and quenched steel is substantiallywholly austenitic. Itis formable and machinable, and, illustr-atively bythese measures, I readily produce such products as turbine blades,rotors, nozzles, turbo-supercharger parts, heat'engine valves asforinternal combustion engines, high-temperature chemical equipment partsforexample to use under high pressure conditions, or rivets, bolts andfasteners, and numerous other products which are subjected to highmechanical stress both "While hot and, cold in corrosive surroundings.Not only do theproducts have high temperature'properties, but they havehigh tensile strength at low temperatures. They are resistant to creepand stress-rupture and are highly resistant tocorrosion in either thehot or cold. condition. .The low-temperature strengthof the steelis-go'od, and I often enhance this as by subjecting the s'teeltohot-cold forging or working as in producing the products.

To achieve precipitation ;heat treatment of atomic slip planes. "Or thecopperpresent may remain in solution andin-hibit coalescence. Re-

.gardless of theory, howevenIfind that my-steel has excellentproperties; The quenched metal has a fine grain structure and isfurtherim-v proved in load-carrying capacity and creep-resistance. Theprecipitated material remains'uncoalesced between the atomic slip planesand ef fective against creep and stresssrupturemf the steelfor-extremely long; periods 'tof time athi 'h temperatures.

The austenitic .phosphorus containing 'steel which I provide thushas,-by virtue of the par-: ticular combination-of elements therein, there-j markable abilit to .be worked ,and fabricated into. products and.articles having hfigh=tem treatment of the metal as during the productmanufacturing operations. The comparative ease of working-thehigh-temperature metal, as

by means of hot forming or hot-cold forming.

operations, is a valuable and practicalaspect from the standpoint ofachieving the.- products and articles.

My turbine equipment, or other products of the steeL-are verysatisfactory for withstanding the vigorous effects of high temperatures,this for example over long periods of use under stress and withoutdifficulty from grain growth or creep or failure by fatigue or rupture.Further, the equipment resists warping and hurtful scaling and corrosiveor scouringattack'by gases at high temperatures."

In a more specific preferred composition, my steel and the products madethereof'contain, in approximate percentages, 0.08 .to 0.15 carbon, 15%to 17.5% chromium, 12%"'to 15% nickel, 0.075% to 0.15% phosphorus,2.75%.".tow13;50% copper, 1.75% to 2.75% molybdenum, anywhere up to1.25% manganese, and the remainder substantially all iron. Such elementsas silicon and sulphur, while usually present, preferably are small inamount.

.As illustrative of the practice of my invention, I provide austeniticchromium-nickel-phosphorus stainless steel having the preferredcomposition just noted, as by producing a melt of the same in aconventional electric steel making furnace of the Heroult type inaccordance with any one of several well-known melting processes havinggeneral applicability to the production of stainless steel such, forexample, as the process generally described in the United States LettersPatent 1,925,182 issued to Alexander L. Feild on September 5, 1933, orthe United States Letters Patent 2,056,162 issued to William B. Arnesson October 6, 193 36, or in the Patent 2,455,073 issued to Donald L.Loveless on November 30, 1948. I work the austenitic steel from ingotsinto billets by hot-rolling or forging at a temperature of about 2250 F.I work the billets into the rough form of such products as rotor bladesand blade supports for gas turbines, ending, for example, with hot-coldforging or rolling operations. Cutting and machining to size areachieved where desired. Also, fabrication by Welding with anoxy-acetylene torch, or by electric are means, employing weld rods forgiving a deposit preferably of the same composition as the parent metal,is undertaken if desired. Following the Working and fabricatingoperations, for example, I anneal the steel so as to put the copper insolution, quench the metal, and thereafter reheat the same to achieveprecipitation heat treatment. The reheating illustratively is atapproximately 1200 F. in a conventional heat-treating furnace, say forabout five hours at temperature to achieve precipitation hardening. Iquench the precipitation-heat-treated articles or products as by coolingin air. Slip interference, accordingly, prevails in thechromium-nickel-phosphorus steel to enhance the load-carrying capacity.

Table I is presented below to illustrate the highly satisfactorylow-temperature tensile properties, and the high temperature creep andstressrupture properties of my austenitic chromiumnickel-phosphorusstainless steels. The tabulated results relate more particularly to oneof my steels which contains about 0.098% carbon, 16.44% chromium, 13.88%nickel, 0.103% phosphorus, 2.44% molybdenum, 3.22% copper, 0.46%manganese, 0.39% silicon, 0.013% sulphur, and the Physical properties ofCr-N i-P'stainless'st eel I A. ROOM TEMPERATURE TENsILE PROPERTIES Ult.Tens. 0.2% Yld. Elong. 2", ifg Str. p. s.1. Str., p. s. 1. per cent fiU:

B. STRESS-RUPTURE PROPERTIES I Fracture Elong. Red. of i 5 Time, 2", perArea,

' Hrs. cent per cent 1, 200 48, 000 30 20. 0 29. 0 l, 200 46, 000 99 15.5 25. 0 1, 200 44, 000 450 9. 5 17. 3 l, 500 18,000 38 5. 0 4. 2 1, 500000 215 5. 0 6. 0 l, 500 13, 000 442 3. 5 4. 2

Thus, it will be seen that there are provided in this invention,austenitic chromium-nickelphosphorus stainless steels and products andarticles of the steel, in which the various objects noted herein,together with many thoroughly practical advantages are successfullyachieved. It will be seen that the steel, products and articles aretough, strong and durable, corrosion-resistant and heat-resistant, andare thoroughly capable of withstanding the effects of high temperaturesand load over long periods of time. It will also be seen that the steelshave great physical strength at low temperatures and are highly usefulunder either high or low temperature conditions.

It will be appreciated that in certain high temperature uses or articlesand products of the steel, reliance may be had upon the prevailing hightemperature to achieve precipitation for enhancing the load-carryingcapacity and resistance to creep.

While the annealed and precipitation-heattreated products offer improvedresistance to creep and stress-rupture, as compared with products of thesteel which have not been subjected to the heat treatment, either thetreated or untrated products have useful high temperature and lowtemperature properties which may be availed upon in the light ofimmediate needs.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative and not as a limitation.

I claim:

1. Austenitic chromium nickel phosphorus stainless steel of good hightemperature properties and good low temperature strength and workingproperties as well containing approximately 0.01% to 0.15% carbon, 12%to 22% chromium, 8% to 21% nickel,, from 0.05% to 0.5% phosphorus, 1.5%to 4% molybdenum, 2% to 4% copper, 0.10% to 2% manganese, and theremainder iron.

:2. Austenit'i'c chrbmium'nick'ei phosphorus stainless steel ofgood hightemperature properties and good low tempera'imre 's'tren gth and workingprop'ertie's 'as wel l containing approxiphosphorus, 2.75% to 3.50%copper, 1.75 to 2.75% molybdenum; up to 1.25% manganese, and

the remainder iron.

3. Age-hardened austenitic chromium-nickelphosphorus stainless steelarticles ofgoodhigh temperature properties, containing approximately0.01% to[ 0.'1'5% carbon, 12% to 22% chromium, 8% to 21% nickel, from0.05% to'0.5% phosphorus, 1.5% to 4% molybdenum, 2% to 4% copper, 0.10%to 2% manganese, and the remainder-iron, the structure of said steelincluding a dispersed finely divided precipitate derived by annealing"and precipitation heat-treatment.

4. Hot cold worked austenitic chromiumnickel-phosphorusstainlesssteelarticles of good high temperature properties containingapproxi- CLARKE; JR:

REFERENCES CITED I ""Ihe' following" references are of record iii thefile of this" patent: I A H I UNITED STATES PATENTS OTHER REFERENCESMetalsand A110ys,-Feb. 1 937, pages-53 to 58.

1. AUSTENITIC CHROMIUM - NICKEL - PHOSPHORUS STAINLESS STEEL OF GOODHIGH TEMPERATURE PROPERTIES AND GOOD LOW TEMPERATURE STRENGTH ANDWORKING PROPERTIES AS WELL CONTAINING APPROXIMATELY 0.01% TO 0.15%CARBON, 12% TO 22% CHROMIUM, 8% TO 21% NICKEL,, FROM 0.05% TO 0.5%PHOSPHORUS, 1.5% TO 4% MOLYBDENUM, 2% TO 4% COPPER, 0.10% TO 2%MAGANESE, AND THE REMAINDER IRON.